The asteroid, catalogued as 2009 BD, is an oddity. Astronomers think it might be what's known as a co-orbital asteroid, "circling the sun in near-tandem with our planet," according to the web site Spaceweather.com.

Another co-orbital asteroid is 2003 YN107, which corkscrewed a path in Earth's vicinity from 1999 through the middle of last year, then finally departed. A handful of other co-orbital asteroids have been found, including one called 2002 AA29 that astronomers described as behaving like a second moon to our planet.

Estimates of the orbit of 2009 BD keep it with about 9.3 million miles of Earth through November, 2010. The rock is about about 33 feet (10 meters) in diameter and will be about 400,000 miles (644,000 km) away at its closest approach Sunday, Jan. 25.